The present invention relates to electrical heating units, and to methods of manufacturing such heating units. In particular, the present invention relates to a combination thermal insulating block and one or more electrical heating elements, and to methods for manufacturing such units.
It is necessary to use some form of thermal insulating material to confine heat, particularly at elevated temperatures. In recent years, thermally insulating panels have been molded which contain lightweight ceramic fibers. Such panels are highly porous, and provide good thermal insulation at relatively low cost. U.S. Pat. No. 3,500,444 to W. J. Hesse et al describes such a panel and a filter molding process for producing such panels. In addition, the Hesse patent discloses electrical heating elements mounted on or adjacent to one of the surfaces of such a panel for use in a domestic or commercial electric range.
A helical electrical heating element partially disposed upon the surface of a panel of molded inorganic refractory fibrous material and partially embedded in the panel has not proven satisfactory for many applications, such as the walls or roof of a high temperature furnace. A helical wire heating element requires support along its length to prevent sagging, particularly at elevated temperatures. Further, the expansion and contraction rates of the heating element and the molded thermal insulating block differ, tending to cause the heating coil to break free from the block of thermal insulating material. The thermal insulating material itself has little structural strength. Accordingly, there have been extensive efforts to develop superior constructions combining electrical heating elements with such molded thermal insulating blocks.
In addition to providing mechanical support for the heating element which is effective throughout the life of the heating element, it is desirable for the heating element to be positioned to provide maximum radiation and convection heat transfer to the work load and to provide the maximum thickness of thermal insulating material between the electrical heating element and the side of the insulating block opposite the heating element. These considerations must be balanced against cost and ease of production.
A combination heating element and thermal insulating panel suitable for use in a high temperature furnace is disclosed in U.S. Pat. No. 4,278,877 to E. R. Werych. This Werych patent discloses oval elongated thermal resistance coils embedded in the panel adjacent to one surface therof with the longitudinal axes of the coils parallel to the surface. In this manner the portion of each oval coil of the heating element remote from the surface is closer to the surface than it would be were the coil cylindrical, but this remote portion of the coil nonetheless will operate at a higher temperature than the portion of the coils adjacent to the surface.
The pending United States patent application of J. Boes and L. Saris, Ser. No. 477,725, now U.S. Pat. No. 4,617,450 entitled A VACUUM FORMED ELECTRICAL HEATING DEVICE AND METHOD OF PRODUCTION discloses a similar thermal panel in which the interior region of the oval heating coils is maintained substantially free of insulating material in order to reduce the temperature of the portion of the heating coil remote from the radiating surface of the panel. In one embodiment of the Boes and Saris application, the heating coils are positioned within the block of thermal insulating material and spaced from the radiating surface of the thermal insulating material, and slots or grooves are provided between the electrical heating coils and the heat radiating surface. This construction has the advantage of retaining the heating coils more securely in the block of thermal insulating material, but still permits the radiant energy and convection from the heating coils to impinge upon the work load. However, the interior portion of the oval heating elements do operate at a higher temperature than the portion of the heating elements adjacent to the radiating surface of the block, thus reducing the capacity and efficiency of the heating panel.
Resistance elements in the form of a rod of resistance material bent in a series of reverse spaced bends to form a flat element are common in the electric furnace art, and such elements have also been mounted on molded ceramic fiber insulating panels. U.S. Pat. No. 4,403,329 of E. R. Werych entitled SUPPORT SYSTEM FOR ELECTRICAL RESISTANCE ELEMENT discloses a pin for insertion in such ceramic fiber panels provided with a clip for engaging one of the bends of such a serpentine resistance element. U.S. Pat. No. 4,299,364 of P. J. Loniello entitled INSULATING MODULE INCLUDING A HEATER ELEMENT SUPPORT also discloses a rod molded in the insulating panel and extending therefrom, the rod being provided with keeper pins for retaining the electrical heating elements adjacent to the surface of the thermal insulating panel. While such mounting devices position the heating element to utilize the radiant and convection heat transfer produced by the heating element, and permit the thermal insulating block to provide substantially maximum thermal insulation, they are costly and require considerable hand assembly work in construction. In addition, the movable parts of such hangers and mounting structures tend to fail under severe use conditions.